Thorough migration steps!

This patch provides thorough migration steps for preparing existing version 2
factories, initializers, and plugin managers to make themselves forwards
compatible with version 3. This includes the following interfaces and abstract
classes:

- `FactoryInterface`
- `AbstractFactoryInterface`
- `DelegatorFactoryInterface`
- `InitializerInterface`
- `AbstractPluginManager`

Author: weierophinney